Emissions regulations for internal combustion engines have become more stringent over recent years. The regulated emissions of NOx and particulates from internal combustion engines are low enough that in many cases the emissions levels cannot be met with improved combustion technologies. Therefore, the use of aftertreatment systems on engines to reduce emissions is increasing. For reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
SCR systems generate ammonia to reduce the NOx. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if there is excess ammonia in the exhaust, ammonia can slip out the exhaust pipe. Ammonia is an extreme irritant and an undesirable emission. Accordingly, slips of even a few tens of ppm are problematic. Additionally, due to the undesirability of handling pure ammonia, many systems utilize an alternate compound such as urea, that vaporizes and decomposes to ammonia in the exhaust stream. Presently available SCR systems treat injected urea as injected ammonia, and do not account for the vaporization and hydrolysis of urea to component compounds such as ammonia and isocyanic acid. As a result, the urea can decompose to ammonia downstream of the SCR causing ammonia slip, and less ammonia may be available for NOx reduction than the control mechanism estimates causing higher NOx emissions at the tailpipe.
SCR systems that utilize urea dosing to generate ammonia depend upon the real-time delivery of urea to the SCR catalyst as engine NOx emissions emerge. Urea dosers have relatively slow physical dynamics compared to other chemical injectors such as hydrocarbon injectors. Therefore, urea doser dynamics can substantially affect an SCR controls system.
Some currently available SCR systems account for the dynamics of the urea dosing and the generally fast transient nature of the internal combustion engine by utilizing the inherent ammonia storage capacity of many SCR catalyst formulations.
One currently available method introduces a time delay at the beginning of an engine NOx spike before urea dosing begins (or ramps up), and a time delay after the NOx spike before urea dosing ends (or ramps down). Ordinarily, an engine NOx spike will cause a temperature increase in the exhaust gas and SCR catalyst, which may result in the release of stored ammonia on the catalyst. This is especially true when engine power output is used as a substitute for directly estimating engine NOx emissions. The ammonia release provides ammonia for reducing engine out NOx while delaying urea injection prevents excess ammonia from slipping out the exhaust. On the NOx decrease, normally the temperature of the engine exhaust and SCR catalyst decrease, and therefore continued urea injection (the delay before ramping down urea injection) provides ammonia to store on the SCR catalyst and recharge the catalyst.
In many ordinary circumstances, the time delay method causes desirable results in the SCR catalyst. However, in some cases the time delay method can produce undesirable results and even responses that are opposite from an optimal response. For example, a decrease in EGR fraction for any reason causes an engine out NOx spike with a decrease in exhaust temperature. In a time delay system utilizing engine-out power as a substitute for NOx emissions, the change will likely be ignored and a standard amount of injected urea will cause an increase in NOx emissions. In a time delay system that recognizes the engine out NOx spike, the system delays injecting ammonia-creating urea. Because the temperature on the SCR catalyst is relatively lower, the amount of NOx—reducing ammonia released from the catalyst is reduced, which results in a NOx emissions increase. At the end of the NOx spike event, the exhaust temperature increases (from restoration of the designed EGR fraction) while the NOx emissions decreases. The SCR catalyst ejects ammonia from the reduced storage capacity while the urea injector continues to add ammonia to the system without NOx available for reduction. Therefore, the system can slip significant amounts of ammonia on the down cycle.
Other currently available systems determine whether the SCR catalyst is at an ammonia storing (adsorption) or ammonia ejecting (desorption) temperature. When the SCR catalyst is storing ammonia, the system injects urea until the catalyst is full. When the SCR catalyst is ejecting ammonia, the system halts injection and allows stored ammonia to release and reduce NOx.
Presently available systems tracking the SCR catalyst temperature suffer from a few drawbacks. For example, the amount of ammonia stored on the SCR catalyst varies with temperature. However, presently available systems assume a storage amount below a specified temperature, and zero storage above the specified temperature. Therefore, the controls may toggle significantly around the specified temperature, significantly overestimate ammonia storage capacity just below the specified temperature, and significantly underestimate ammonia storage capacity just above the specified temperature. Such systems utilize the “normalized stoichiometric ratio” (NSR) to determine baseline urea injection, but do not account for variances in the NOx composition and NH3 to isocyanic acid ratio of the urea when determining the NSR. Further, such systems do not account for the incomplete vaporization and hydrolysis of urea that occurs in many systems and may therefore not inject sufficient urea to reduce NOx and/or provide the desired ammonia for storage.
Also, many known SCR systems do not utilize an ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst to convert at least some ammonia slipping from the SCR catalyst to N2 and other less harmful compounds. For those conventional SCR systems that do employ an AMOX catalyst, the operating conditions and conversion capability of the AMOX catalyst are not factored into the reductant dosing rate, ammonia storage control, ammonia slippage control, and NOx conversion efficiency feedback of such systems.
Ammonia slippage also poses a problem for NOx sensor technology, which has been found to exhibit ammonia cross-sensitivity. Thus ammonia slippage could be erroneously interpreted as NOx emission, thereby calling for increased reductant dosing via a feedback control loop. The resulting positive feedback in that case would have the undesirable effect of further increasing rather than decreasing the ammonia slippage.